It has become common practice in the field of exploration for oil, gas, and other valuable minerals, to perform seismic exploration in the world's oceans. One common method of seismic exploration is to provide an exploration vessel with one or more compressed air "guns" which comprise tanks for the storage of highly compressed air which may be rapidly released so as to produce an acoustic wave in the water (a "shot"). The acoustic wave thus produced travels through the water to the bottom of the sea, continues through the sea bed and is partially reflected at the interfaces between layers of rock of different densities, back towards the surface where it may be detected by piezoelectric detectors towed behind the vessel. If a plurality of detectors are towed behind the vessel, the difference in time of detection of the reflection from the same interface can be used to calculate the depth of the interface. By providing a plurality of parallel detections in the same general region, therefore, the shape of the interface with respect to the surface of the sea can thus be calculated; and since certain interface formations are known to be conducive to the formation and/or accumulation of oil and gas, these seismic signals are a valuable tool in oil and gas exploration.
The general subject of seismic prospecting for oil and the processing which can be performed on the data thus gathered to lead to better information is the subject of numerous reference works in the field. The present invention is directed at means for efficient storage of the seismic data thus gathered.
It will be appreciated by those skilled in the art that seismic data recordation requires immense amounts of digital data storage since vast amounts of data are generated in the seismic exploration process. In the preferred embodiment discussed below, it will be disclosed that even using the most modern up-to-date high-data-density magnetic storage tapes, an entire reel of tape is filled with seismic data within about eleven minutes of seismic exploration running time. This is a vast quantity of data indeed and it therefore behooves the data storage system designer to store the data as economically as is possible and to prepare it as fully as practicable while detecting it for the processing which is eventually to be performed thereon on a land-based computer so as to save all possible fractions of computer time in advance. This motivation--to save as much time and to store the data as economically as possible--forms the framework within which the present invention is made.
Therefore, it has been the aim of the prior art to provide seismic data storage systems which are so designed as to arrange the data for economic computation thereon when the magnetic tapes containing the data are eventually returned to a home base computer for complete analysis. For example, in commonly assigned U.S. Pat. No. 4,016,531 to Cook et al, means are disclosed for rearranging the data from the serial by time of detection format in which the data is generated into a serial by trace format which is more economic for computation at a home base computer. Similarly, in commonly assigned U.S. Pat. No. 4,084,151 to Penner relatively simple computations are performed on the data while it is being reordered in accordance with the scheme of U.S. Pat. No. 4,016,531 referred to above. Both these disclosures are hereby incorporated by reference in the present application; their subject matter forms a part of the system within which the present invention is also designed to operate.
It will be appreciated by those skilled in the art that the more seismic data available from a given area, the better use can be made of the data by experienced geophysicists when analyzing the data in the search for oil. Therefore, it has been and remains an aim of the art to provide more and more acoustic energy detectors in the streamer towed behind the exploration vessel so that more data can thus be generated and more accurate results obtained. However, the provision of additional acoustic detectors means that more traces can be made available for recording. As more data is generated, the faster the computers on board the vessel must operate to rearrange the data, as well as to perform scaling and other arithmetic operations on it; it has been necessary to provide additional computing power on board the vessel in order that additional data generated by the additional acoustic energy detectors can be dealt with.
An additional aim of the art has been to provide automatic means for recording parameters relating to the circumstances under which the data was generated on the same tape on which the data itself is recorded; that is to say, it is desirable to provide a "header" for each record of data of an acoustic "shot" so that the tape may be self-identifying. Desirably each seismic record should include all navigational data, including the vessel's speed and heading, the general weather conditions, and any source of additional acoustic or electronic noise which might be detected. It would also include information as to the number of acoustic energy detectors being streamed behind the boat and the curvature of the streamer with respect to the vessel. Furthermore, it is desirable that the seismic record be provided with a space for operator comments; for example, the operator might note that noise was being generated by another vessel in the vicinity of the exploration vessel. Clearly it would be desirable to record this information directly on the tape so that it could be readily removed in the processing of the data.